ColorSpace

sRGB (standard Red Green Blue) is an RGB color space that HP and Microsoft created cooperatively in 1996 to use on monitors, printers, and the Internet. It is often the “default” color space for images that contain no color space information, especially if the images’ pixels are stored in 8-bit integers per color channel.

In the 1920s, W. David Wright and John Guild independently conducted a series of experiments on human sight, from which they tried to determine the colors in human vision, and give it a mathematical basis. The results of these experiments defined the CIE 1931 RGB color space which became the basis for the trichromatic CIE 1931 XYZ color specification.

In 1931, based on the results of the CIE RGB version of the human eye, CIE mathematically determined a theoretical color space called the CIE 1931 XYZ color space to map out all of the colors perceived by the human eye. It is considered to be one of the most accurate color spaces; thus, it is commonly used for color management and conversions.

X, Y and Z are extrapolations of RGB created mathematically to avoid negative numbers and are called Tristimulus values. Y means luminance, Z is somewhat equal to blue, and X is a mix of cone response curves chosen to be orthogonal to luminance and non-negative.

In CIE xyY, Y is the luminance and x and y represents the chromaticity values derived from the tristimulus values X, Y and Z in the CIE 1931 XYZ color space. CIE xyY is just another way to represent CIE 1931 XYZ.

The CIE 1960 UCS color space is another name for the (u, v) chromaticity space devised by David MacAdam. The CIE 1960 UCS does not define a luminance or lightness component. Today, the CIE 1960 UCS is mostly used to calculate correlated color temperature, where the isothermal lines are perpendicular to the Planckian locus.

The CIE 1964 U*V*W* color space is based on the CIE 1960 UCS. Wyszecki invented the UVW color space in order to be able to calculate color differences without having to hold the luminance constant. He defined a lightness index W* by simplifying expressions suggested earlier. The chromaticity components U* and V* are defined such that the white point maps to the origin. This arrangement has the benefit of being able to express the loci of chromaticities with constant saturation.

CIE 1976 L*a*b* is a color space in which L is lightness and a and b are chromaticity components, with the difference that the color values are far more than the human gamut. It was designed to be perceptually uniform with respect to human color vision, meaning that the same amount of numerical change in these values corresponds to about the same amount of visually perceived change.

The CIE 1976 L*u*v* color space is a simple-to-compute transformation of the CIE 1931 XYZ color space, but which attempted perceptual uniformity. It is extensively used for applications such as computer graphics which deal with colored lights.

The CIELCh° color space is a cylindrical representation of the CIELAB or CIELUV cube color space, where C* is the chroma and h° is the hue. The L* lightness value remains unchanged.

The LCh color space is not the same as the HSV, HSL or HSB color models, although their values can also be interpreted as a base color, saturation and lightness of a color. The HSL values are a polar coordinate transformation of what is technically defined RGB cube color space. LCh is still perceptually uniform.

The Hunter Lab color space, defined in 1948 by Richard S. Hunter, is another color space sometimes referred to as “Lab”. Like CIELAB, it was also designed to be computed via simple formulas from the CIE 1931 XYZ space but to be more perceptually uniform than CIE 1931 XYZ.

Hunter named his coordinates L, a, and b; the CIELAB space, defined years later in 1976, named its coordinates L*, a*, and b* to distinguish them from Hunter’s coordinates.